Excessive Data Exposure (JavaScript / TypeScript)

ID

excessive_data_exposure_jsts

Severity

high

Remediation Complexity

medium

Remediation Risk

high

Remediation Effort

medium

Family

API3:2023 - Broken Object Property Level Authorization

CWE

CWE-213, CWE-200

Resource

data_exposure

Language

Express, NestJS, Koa, Fastify, Hono

Description

Walks each endpoint’s response model and classifies every field into one of three confidence tiers, based on whether the field is sensitivity-tagged (PII / PCI / PHI / credentials by the sensitivity classifier) and whether the same field name appears in the request:

  • HIGH — sensitive AND not referenced in the request. The caller never asked for the field; returning it is over-fetch.

  • MEDIUM — sensitive AND referenced in the request. Possibly a legitimate field-by-field update — surfaced for review.

  • LOW — not sensitivity-tagged AND not referenced in the request. Mild signal of over-fetching.

Only the HIGH tier fires by default. The detector is per-language because the framework idioms differ — JSON-annotation models in Java / C#, dataclass / Pydantic models in Python, plain-object / class-transformer in JS/TS, struct tags in Go, Eloquent / Symfony serializer groups in PHP.

Rationale

Excessive Data Exposure is the read side of OWASP API3:2023 — the API returns more than the client needs, because the implementation returned a persistence model directly and the persistence model has more fields than the contract. The risk is twofold:

  • Direct data leak — the extra fields contain credentials (passwordHash, apiToken), PII (email, phone, ssn), or PCI (cardLastFour, accountNumber). Anyone reading the response sees them.

  • Object-property authorization bypass — even if the endpoint is authorised to return the object, individual fields on that object should be scoped. A "GET my profile" endpoint authorised for the user should not expose the user’s isAdmin flag, lastLoginIp, or internalNotes. API3 separates these from API1 (object-level) for exactly this reason.

The pattern is endemic in single-page-app backends, where the same User / Order / Patient entity is reused across all read endpoints, with the client deciding which fields to show. The "client filters out the bad fields" defence does not work — anyone can read the network response.

A susceptible Express / Mongoose route might look like this:

const userSchema = new mongoose.Schema({
  username: String,
  email: String,         // PII
  passwordHash: String,  // CREDENTIAL
  ssn: String,           // PII
  isAdmin: Boolean,      // PRIVILEGE
});
const User = mongoose.model('User', userSchema);

app.get('/users/:id', async (req, res) => {
  const user = await User.findById(req.params.id);
  res.json(user);
});

res.json(user) serialises the entire Mongoose document — including passwordHash, ssn, and isAdmin.

Remediation

The fix is to return a dedicated response model per operation, narrow to the fields the operation actually needs to expose. Specific per-language patterns are in the language pages.

Beyond per-route DTOs:

  • If the sensitivity tag is wrong for your project — for example, an email-newsletter app whose User.email field is intentionally public — exclude the field via per-project sensitivity overrides rather than suppressing the finding.

  • Pair this detector with pii_leak_in_response (focuses on PII / PCI / PHI specifically, with stricter severity).

  • Server-side response filtering is the only effective control. Anything that depends on the client hiding fields is not a remediation.

Here is a revised handler with an explicit projection:

const toPublic = (u) => ({ id: u.id, username: u.username });

app.get('/users/:id', async (req, res) => {
  const user = await User.findById(req.params.id, 'username').lean();
  if (!user) return res.sendStatus(404);
  res.json(toPublic(user));
});

The projection 'username' selects only the needed column at the database layer, and toPublic is the single place where the wire shape is defined. Adding a sensitive field to the Mongoose model no longer leaks it.

Framework-specific patterns:

  • NestJS: @SerializeOptions({ strategy: 'excludeAll' }) on the controller + @Expose() on the DTO fields that should be returned. Use ClassSerializerInterceptor to enforce the projection globally.

  • TypeScript: type the response as a narrow interface (type PublicUser = Pick<User, 'id' \| 'username'>) — compile-time check that you don’t accidentally send extra fields.

Configuration

The detector accepts:

  • minConfidence — the minimum confidence tier that fires. Default high. Set to medium to include "sensitive AND referenced in request" findings. Set to low only for benchmark / audit runs (very noisy).

The set of sensitivity tags (PII / PCI / PHI / credentials) is configured globally on the sensitivity classifier, not per-detector.

References